foo.cc: In function 'int main()':
foo.cc:16:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   16 |         for(int j=0; j<a[i].size(); j++)
      |                      ~^~~~~~~~~~~~

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